1. Field of the Invention
The present invention relates to an anti-reflection film used in photolithography during the steps of manufacturing a semiconductor device and a method of manufacturing the same.
2. Description of the Background Art
In every step of manufacturing a semiconductor device, photolithography is used in which a resist film is formed in a predetermined pattern by using a photomask. Then, the pattern of the resist film is transferred to a silicon wafer.
Description will be given to the step of forming a word line as a step of manufacturing a conventional DRAM (Dynamic Random Access Memory) hereinafter.
Referring to FIG. 13, an insulating film 51 is formed on a semiconductor substrate 50. A polysilicon film 52a is formed on insulating film 51. An insulating film 52b is formed on polysilicon film 52a. On insulating film 52b formed is a resist film 53. A photomask 54 having a predetermined pattern formed thereon is disposed above resist film 53. An exposure light 55 such as an i-line (.lambda.=365 nm) or krF light (.lambda.=248 nm) is directed to resist film 53 from above photomask 54 therebetween.
Referring to FIG. 14, the resist film 53 is exposed by exposure light 55 along the pattern of the photomask. Thereafter, prescribed portions of the resist film 53 are removed by development.
Referring to FIG. 15, etching of insulating film 52b and polysilicon film 52a is carried out by dry etching 57 with resist film 53 used as a mask. Referring to FIG. 16, by removing resist film 53, a word line 52 is completed.
When there is a stepped portion in an underlying film such as a word line, however, exposure light is reflected by the stepped portion, causing a problem that a resist film of a desired shape cannot be formed. Description will be given to the problem hereinafter with reference to FIG. 17. FIG. 17 is a cross section in which an element isolation region 56 is formed on semiconductor substrate 50.
Element isolation region 56 and insulating film 51 are formed on semiconductor substrate 50. There is a stepped portion 56a at a joint of element isolation region 56 and insulating film 51.
Polysilicon film 52a and insulating film 52b are formed along the surface of element isolation region 56 and insulating film 51. Insulating film 52b has a stepped, portion 52c along stepped portion 56. Resist film 53 is formed on the upper surface of insulating film 52b. Resist film 53 has a flat surface. Photomask 54 having a predetermined pattern is disposed above resist film 53.
Exposure light 55 such as the i-line (.lambda.=365 nm) or krF light (.lambda.=248 nm) is directed to resist film 53 from above, with photomask 54 therebetween. Referring to FIG. 18, exposure light 55 reaching insulating film 52b is reflected by the surface of insulating film 52b. An exposure light 55a directed to a flat portion of the surface of insulating film 52b is reflected in the incident direction. Exposure lights 55b, 55c, 55d and 55e directed to stepped portion 52c of insulating film 52b, however, are reflected in accordance with a tilt angle of stepped portion 52c as shown in the figure. Reflected lights 55b, 55c, 55d and 55e of exposure light 55 expose a region of a resist film 53c which is not to be exposed originally.
Referring to FIG. 19, resist film 53 formed as described above has a chipped portion as shown in the figure. If polysilicon film 52a and insulating film 52b are etched by dry etching 57 using resist film 53, portions of polysilicon film 52a and insulating film 52b not to be etched are etched as shown in FIGS. 20 and 21. As a result, a word line 52 having a predetermined shape is not formed.
Referring to FIG. 22, provision of an anti-reflection film 58 on insulating film 52b for preventing reflection of exposure light is a known technology in order to eliminate the above-described problem. Reflectance required in the step of general photolithography is 10% or less, and anti-reflection film 58 primary of TiN has conventionally been used.
Description will now be given to a method of forming word line 52 when anti-reflection film 58 of TiN is used.
Referring to FIG. 22, anti-reflection film 58 of TiN is formed on the upper surface of insulating film 52a by using a sputtering method or the like. Resist film 53 is formed on anti-reflection film 58. Photomask 54 having a predetermined pattern is disposed above resist film 53.
Referring to FIG. 23, exposure light 55 such as the i-line (.lambda.=365 nm) or the krF light (.lambda.=248 nm) is directed to resist film 53 from above, with photomask 54 therebetween. At this time, exposure light 55 reaching insulating film 52b is scarcely reflected by stepped portion 52a because of anti-reflection film 58. This enables exposure in accordance with a pattern of resist film 53 and resist film 53 of a desired shape can be obtained.
Referring to FIG. 24, with resist film 53 used as a mask, anti-reflection film 58, insulating film 52b and polysilicon film 52a are etched by etching 57 to form word line 52. Then, referring to FIG. 25, by removing resist film 53 and anti-reflection film 58 remaining on word line 52, word line 52 shown in FIG. 26 is completed.
However, a TiN film used as the anti-reflection film has a problem as in the following.
(1) It is difficult to dry-etch the TiN film since a titanium atom has a low vapor pressure to any compound.
(2) Taking the above problem (1) in consideration, a TiN film is formed on a Si material. When the TiN film is etched to a prescribed shape by wet etching using H.sub.2 SO.sub.4 /H.sub.2 O.sub.2, the Si material is not etched at all. However, since side etches are caused in the TiN film by wet etching, it is difficult to etch the TiN film to a prescribed shape.
(3) Since titanium has a level in a band gap in silicon, leak current and the like are increased, resulting in degraded characteristics of Si as a semiconductor.